1. Field of the Invention
The present invention relates to electrically driven water purification devices and, in particular, to novel sealing means to facilitate sealing of such devices.
2. Description of the Related Art
Water purification devices of the filter press type which purify water by electrically driven membrane processes, such as electrodyalisis or electrodeionization, comprise individual chambers bounded by ion exchange membranes. Typically, each of the chambers is defined on one side by a membrane disposed to the preferential permeation of dissolved cation species (cation exchange membrane) and on an opposite side by a membrane disposed to the preferential permeation of dissolved anion species (anion exchange membrane).
Water to be purified enters one chamber commonly referred to as a diluting chamber. By passing a current through the device, electrically charged species in the diluting chamber migrate towards and through the ion exchange membranes into adjacent chambers commonly known as concentrating chambers. As a result of these mechanisms, water exiting the diluting chamber is substantially demineralized. Electrically charged species which permeate through the ion exchange membranes and into a concentrating chamber are flushed from the concentrating chamber by a separate aqueous stream flowing through the concentrating chamber.
To this end, the above-described devices comprise alternating diluting and concentrating chambers. In addition, cathode and anode chambers, housing a cathode and an anode respectively therein, are provided at the extreme ends of such devices, thereby providing the necessary current to effect purification of water flowing through the diluting chamber.
For maintaining separation of associated cation and anion exchange membranes, spacers are provided between the alternating cation and anion exchange membranes of the above-described water purification devices. Therefore, each of the diluting chambers and concentrating chambers of a typical electrically-driven water purification device comprise spacers sandwiched between alternating cation and anion exchange membranes.
To prevent any appreciable leakage from diluting chambers and concentrating chambers of such devices, the above-described arrangement of spacers sandwiched between ion exchange membranes must form a substantially water-tight seal. To this end, the spacers and the ion exchange membranes are pressed together and fixed in position with known connectors. Unfortunately, this alone has not provided adequate sealing characteristics.
Various attempts have been made to improve the sealing characteristics of electrically driven water purification devices. For instance, it is known to use an adhesive to bond the ion exchange membranes to either side of a spacer. Unfortunately, as a result of exposure to typical operating conditions, the seal formed thereby is prone to leakage, thereby causing the loss of valuable product water. This arises from the intrinsic moisture permeability of the ion exchange membranes and because of poor mechanical sealing characteristics.
Alternatively, it is known to provide spacers having resilient o-ring type sealing members on either side of the spacer for engaging an opposing planar surface of adjacent spacers. A cation exchange membrane and an anion exchange membrane are pressed against opposite sides of the spacer and function as permselective barriers. In this arrangement, the spacer, its o-ring type sealing member, and ion exchange membranes define a space wherein ionic species in aqueous fluid media contained therein can migrate in a direction substantially orthogonal to the plane of the spacer and permeate through either of the ion exchange membranes. Unfortunately, during assembly of the device, it is known to be difficult to maintain ion exchange membranes in a desired alignment relative to associated spacers. Further, during operation and consequent exposure to relatively high internal pressure or differential pressures within the device, ion exchange membranes may move and become displaced from a desired position relative to their associated spacers. Failure to maintain such a desired position may compromise the sealing of the associated chamber.
In an attempt to limit movement of ion exchange membranes during assembly of the water purification device, ion exchange membranes have been provided with alignment holes which receive fixed rod-like structures. However, this provides a further potential source for leakage and, therefore, compromises sealing of the device.
The material of construction of known spacers is also known to be detrimental to the sealing characteristics of this arrangement. To facilitate mass production by injection moulding, spacers are typically manufactured from thermoplastic materials, such as polypropylene. Unfortunately, such thermoplastic materials are prone to stress relaxation or compression set. As a result, over time, because of exposure to the relatively high internal pressures, such spacers, and particularly their o-ring type sealing members, lose their resiliency, thereby compromising their ability to maintain adequate seals with adjacent surfaces.